What size will I be if started with small D and put in 800cc naltrelle style 20 implants?

Thank you for the question. There is no accurate correlation between size of breast implant and cup size achieved.
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "D or DD cup”  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
 The use of computer imaging technology may also be very helpful during this communication process.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.
I hope this (and the attached link, dedicated to larger breast augmentation outcomes) help.

How big you will be after placing 800cc implants depends in part on how on your height, weight, breast width, torso width and other factors.  However, 800 cc implant will increase you by a minimum of two cup sizes and more likely 3+ cup sizes.  800cc implants are obviously quite large and not recommended in the average patient for breast augmentation in my opinion.  Long term negative consequences of large implants include stretching of the breast skin and a suboptimal long term result with significant increased risks and need for revision surgery.